The present invention relates to a color imaging system wherein the average of one or more colors is used to modify the saturation of another color.
Blooming is a problem resulting from constraints in intensity and misalignment tolerances as found in many imaging scenarios and types. Much of the art solving blooming problems is directed primarily toward video and CCD type cameras. However, color imaging systems such as video display monitors and print systems can also exhibit blooming problems in their output. By "blooming" we are referring to shapes in an image which exceed their intended edges or boundaries and are thus larger than desired in a manner objectionable to the human eye. Examples of blooming include, line growth and narrow white space fill in. Two important parameters affecting blooming in color systems are alignment, and saturation or intensity. The consequences of these two parameters are manifest in both color image display and color image print systems. The method and apparatus disclosed here are applicable in both such systems as is apparent to one skilled in the art. However, for purposes of discussion the focus will be upon printing systems. In particular, the discussion will focus on xerographic, electrophotographic or any other electrostatographic printing processes.
In a typical electrostatographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize its surface. The charged portion of the photoconductive member is exposed by a writer, for example: by light from the image of the original document, or by a raster output scanner (ROS) in the case of an electronically stored image. Exposure of the charged photoconductive member selectively dissipates charge in the irradiated areas. This renders a record, with the electrostatic latent image on the photoconductive member corresponding to the informational areas contained in the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing developer material into contact with the surface of the photoconductive member. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image by electrostatic force thus forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
The foregoing generally describes a typical black and white electrostatographic printing machine. With the advent of multicolor electrophotography, it is desirable to use an architecture which comprises a plurality of image forming stations. One example of the plural image forming station architecture utilizes an image-on-image (IOI) system in which the photoreceptive member is recharged, reimaged and developed for each color separation, followed by a single transfer of all colors to paper. This greatly simplifies the transfer system, hence the system architecture, in comparison with a traditional system where each color is transferred to paper separately. As such, IOI machines can be single pass or multipass. In single pass IOI machines the charging, imaging, developing and recharging, reimaging and developing, etc followed by transfer to paper, are done in a single revolution of the photoreceptor. This requires multiple charging and imaging stations. In multipass IOI architectures, each color separation is formed with a single charging and imaging station but with different development stations, though still with a single transfer operation of all the colors. The single pass architecture offers a potential for much higher throughput.
In an IOI machine the second, third and fourth exposure or imaging stations need to image through prior developed toner layers, hence their intensities need to be increased. With IOI all the desired toner colors are built up on the photoreceptor before transferring to a page and fusing. The first color toner is applied in the same manner as black and white or single color electrophotography. Everywhere, the first color is applied directly to the photoreceptor. Thereafter one or more subsequent colors are applied over the first color. Where a color blend is desired, the second color toner is applied superimposed over the first color toner. Where only the second color is desired however, it is applied directly upon the photoreceptor. The result is that a subsequent color toner layer develops, in some places over the prior toner layer, and in other places directly upon the photoreceptor. This requirement creates a problem in the exposure step prior to this second toner development step. For that exposure step, the raster output scanning (ROS) level if set to the same exposure level as before, will not achieve the same photoreceptor charging levels because the laser needs to image through the first toner layer. This ultimately results in an insufficient mass of toner with attending color and saturation problems in the print. Thus there is a need to increase the ROS exposure level. However, such an increase, when applied to areas where there is no prior developed toner on the photoreceptor, means the exposure level is now too high, and this causes a "blooming" problem (i.e., line growth and narrow white space fill in).
By way of illustration, consider an IOI machine where the colors are developed in the order of yellow, magenta, and cyan. Current practice in IOI xerography calls for high ROS exposures for the cyan separation to discharge the photoreceptor to an equal voltage under magenta and yellow toner and bare photoreceptor. If two exposure powers are available, one for bare photoreceptor and one for areas with previously developed toner, one is freed from the constraint of over exposure because the bare photoreceptor exposure can be independently controlled so as to be equal to the exposure found under toner layers. This is disclosed with U.S. patent application Ser. No. 08/786,611 assigned to the present assignee and incorporated by reference herein, which teaches that such an approach is electrostatographically feasible and shows the image quality benefits which result. In that particular implementation of a multiple exposure technique, transfer of information from the previous channels to control the exposure levels when printing the next separation are utilized on a pixel by pixel basis. However, such an approach relies on near perfect alignment at each station as the photoreceptor is exposed for each color. Where misalignment exceeds one pixel width in amount, which is typical, blooming effects again become manifest. Blooming as resulting from constraints in intensity and misalignment tolerances is a problem found in many imaging scenarios and types.
U.S. Pat. Nos. 5,450,211 and 5,561,743, address as a cause of blooming the change with temperature of focal distance and attendant change of magnification (change in beam diameter). Disclosed is a copying machine for forming an image by a scanning laser beam which is emitted from an optical system on a photosensitive member. Photosensors are provided at optically equivalent positions to a beginning portion and an end portion of a scanning line on a photosensitive member. A scanning time of a laser beam in the main scanning direction is measured by these photosensors. The measured time and a standard time for a copy magnification set by an operator are compared, and a correction value is calculated, and at the same time, magnification in the main scanning direction is corrected using the correcting value. Also, focusing (adjusting of the beam diameter) is carried out using the photosensors. Each of the photosensors contains a photoelectric transfer element, and a beam which comes through a slit is incident to the element. Each photosensor is provided with two slits. One is perpendicular to the main scanning direction, and the other is inclined to the main scanning direction. The beam diameter in the main scanning direction is adjusted based on detection of the beam which comes through the slit which is perpendicular to the main scanning direction. The beam diameter in the sub scanning direction is adjusted based on detection of the beam which comes through the slit inclined to the main scanning direction.
Another approach to blooming problems is found in U.S. Pat. No. 5,519,815 relating an image processing method for reducing the amount of marking material required to print a colored image, and avoid problems common to the use of excessive amounts of ink. In a device for processing color images preparatory to printing, it provides a method of reducing marking material coverage in text and line art areas of secondary colors including the steps of: a) determining the locations of text and line art color pixels having excessive marking material coverage; b) upon determining the locations of color pixels having excessive marking material coverage, processing separation pixels to turn OFF a predetermined portion of the separation pixels corresponding to color pixels having excessive marking material coverage; and c) to prevent artifacts from occurring in the pixel reduction step, processing a given area of separation pixels in an order which tends to randomize the turn OFF effect.
U.S. Pat. No. 5,296,877 describes a multicolor printing process in which the shape and intensity of an imaging beam are changed when imaging a portion of a photoconductive element upon which toner has been deposited. Disclosed is a method of forming a multicolor image and apparatus capable of reducing the deviation between successive toner images of different colors and enhancing dot reproducibility. In the event of the second and successive sequences of steps included in a single multicolor image forming process, the intensity of a laser beam is controlled together with a beam diameter or an emission pulse width. This control is such that, when the laser beam scans part of the photoconductive element where a toner image was formed by the preceding sequence of steps, the laser beam forms a dot of substantially the same size in both the part where a toner image is present and part where a toner image is absent. This is performed in matching relation to the interception and scattering characteristics of the laser beam due to the toner image.
U.S. Pat. No. 5,252,995 describes a system in which the boundaries are enhanced between different color areas by varying the laser power. A multi-color, electrostatic, laser printing system employs a method for enhancing color representations at boundaries of pixels with different colors. The system employs a charged electrostatic surface that is selectively discharged by application of a modulated laser beam. The system performs the steps of: identifying a pixel of a first color that bounds a pixel of a second color, the first color being a secondary color; decreasing applied laser power to the electrostatic surface at the secondary color pixel site in the process of applying underprint and overprint toners to produce the secondary color; and inhibiting the decrease of applied laser power when applying the overprint toner if the second color pixel is a primary color that is the same as the color of the overprint toner of the secondary color pixel. A similar inhibition action occurs when an isolated primary color pixel or a thin pixel line is found so as to assure proper development.
U.S. Pat. No. 5,023,632 describes a method to vary the exposure between black and color toners by examining neighboring pixels around a target pixel. The logic applied here is to increase the exposure amount when all eight reference spots are in a non-exposure state, but use a reference exposure amount in any other situation. Disclosed is an electrophotographic color image forming method whereby black, yellow, magenta and cyan toner images are superimposed on a photoconductor to transfer to paper, exposure being changed between black and color in a method in which development of black is performed by a contact developing method, and development of color by a direct current electric field projection developing method. Also, in exposure for color, the exposure amount is adjusted between highlight and shadow areas. A full color image of high saturation with good balance between black and color components is reproduced by changing the exposure amount between black and color. It is also possible to obtain a color image with good reproducibility of highlight areas as well as shadow areas.
All of the background art cited above is hereby incorporated by reference.
As may be noted from the discussion above, while there are many approaches to the problem of blooming, none of them satisfactorily address misregistration or misalignment in combination with intensity. Furthermore, a two-level exposure control is too coarse an adjustment, as the compensation needed for imaging through a yellow toner layer is in general different from that required for imaging through a magenta toner layer, which is yet again different from that compensation required for imaging through the combination of both the yellow and magenta toner layers together. Therefore, it would be desirable to have a method and apparatus which solves blooming problems, by allowing appropriate adjustment of intensity without suffering from misalignment effects.